Operation of furnace for the electrolytic fusion recovery of aluminum



Dec. 29, 1970 w, c p -r m ETAL OPERATION OF FURNACE FOR THE ELECTROLYTIC FUSION RECOVERY OF ALUMINUM Y 2 Sheets-Sheet 1 Filed April 4, 1968 INVENTQRS.

5 Gnzde/ ATTORNE 5.

Doc. 29, 1970 w, CAPlTAlNE ETAL 3,551,308

OPERATION OF FURNACE FOR THE ELECTROLYTIC FUSION RECOVERY OF ALUMINUM Filed April 4. 1968 2 Sheets-Sheet 2 INVENTORS. fl OAFGA/VG CAP/7mm:- Ayn/gym 5/4/90 United States Patent 3,551,308 OPERATION OF FURNACE FOR THE ELEC- TROLYTIC FUSION RECOVERY OF ALUMINUM Wolfgang Capitaine and Martin Bard, Rheinfelden, Germany, assignors to Swiss Aluminium Ltd., Chippis, Switzerland, a joint-stock company of Switzerland Filed Apr. 4, 1968, Ser. No. 718,704 Claims priority, application Switzerland, Apr. 7, 1967, 4,990/ 67 Int. Cl. C22d 3/12, 3/02 US. Cl. 204-67 9 Claims ABSTRACT OF THE DISCLOSURE Methods and apparatus for the elimination of the anode effect in furnaces for the recovery of aluminum by fusion electrolysis which involve piercing the crust on the surface of the electrolyte by mechanically driven tools and supplying gas through ducts located in the tools. The pressure of the gas may be varied at different stages.

SUMMARY OF THE INVENTION In the production of aluminium by electrolysis of alu mina in cryolite, the so-called anode effect occurs when the bath becomes impoverished in its alumina content. This effect can be recognised by the fact that the voltage of the electrolytic cell suddenly increases very markedly (from the normal voltage of about 4 v. to about 30 v.). According to attempted explanations given heretofore, this increase in voltage is caused, inter alia, by the fact that due to the reduction in the concentration of alumina, gas bubbles form below the anode, remain attached thereto and reduce the wetting of the anode by the electrolyte, or by the fact that a barrier layer of changed chemical composition is formed in the zone of the anode surface. As a result, direct contact between the anode carbon and the electrolyte ceases and, after a critical voltage has been reached, electrical connections are established by way of numerous discharge paths which lead through the layer of gas or barrier layer.

The anode effect is eliminated by supplying the gas film or barrier layer below the anode by using movements of the bath which are produced by mechanical or pneumatic means. The elimination of the anode effect is therefore normally composed of two successively performed opera tions which are partly carried out manually:

(1) Breaking of the crust and introduction of alumina into the bath in the electrolytic cell. As is known, this operation is generally mechanised.

(2) Removal of the gas film below the anode by temporarily introducing wooden rods, which evolve gases and develop turbulence in the cryolite bath, or by passing compressed air in below the anode by means of a pipe. Both the wooden rods and the gas inlet pipe must be held and moved to and fro in the melt by hand; each of these tools can therefore only enter the melt after the crust has been open.

In this method of eradication employed in practice, the anode effect persists for about 3 to 10 minutes, because after the optical or acoustic signal has been given an operator must first set a crust breaker in operation, drive it to the appropriate furnace (in some cases this may mean long distance where large sheds are involved) and then as described, break the crust. Only after this can the 3,551,308 Patented Dec. 29, 1970 ice anode effect be eradicated by means of wooden rods or compressed air.

Even if by chance two or more cells show an anode effect at the same time, the operator can only treat one cell after the other with his crust breaker, so that in the case of the lastcells to be treated the anode effect lasts even longer. Due to the increased voltage, considerable energy and substantially more electrolyte and anode carbon are consumed during the anode effect than in normal operation.

Considerable efforts have therefore already been made to keep the duration of the anode effect brief, or anticipate it by means of an advance indication, by already staving in the crust of the electrolytic cell beforehand. Through the advance indication (for example by means of a costly computer) and suppression of the anode effect, the energy consumption can be reduced, but the method and the expenditure of time have remained the same and, consequently, automatic eradication of the anode effect has not previously been possible.

On the other hand, it is known that the breaking of the crust in order to introduce alumina into the bath is not only carried out after an anode effect occurs, but that it is expedient to perform this operation one or more times between two anode effects, so as to operate the furnace at as constant a voltage as possible. Thus, the crust is broken at intervals of, for instance, 2 to 4 hours. So as to produce the anode effect about once a day, the addition of alumina is reduced by taking suitable steps on some of the occasions when the bath is serviced.

It has also already been proposed that the mechanical breaking of the crust be automated, both on the servicing of the furnace carried out after an anode effect and on the servicing thereof carried out between two anode effects, and that the increase in voltage occurring before or during the anode effect be used as an initiating means for the remedial measures. Thus, it has also been proposed that the anodes be divided into two half groups, one on each side of an interspace in the longitudinal axis of the furnace, and that a mechanically driven crust-breaking apparatus extending over the entire length of the anode zone be arranged in this interspace. To eliminate any anode effect, manually performed agitation with a wooden rod or manually performed introduction of compressed air by means of a pipe has nevertheless remained necessary, so that the basic method and the necessary expenditure of time have remained substantially the same. As the interspace is relatively narrow and, moreover, access thereto is difiicult at the end of the furnace because of the supporting pillar for the anode beam which is located at this point, it has nevertheless been preferred in practice to break the crust also at at least one of the longitudinal sides of the furnace and effect the agitation of the electrolyte from this position, which, after all, is also timeconsuming.

Acording to the invention, in a method of operating a furnace for the recovery of aluminium by fusion electrolysis, the anode effect is eliminated by piercing the crust on the surface of the electrolyte with at least one mechan ically driven tool and supplying gas to the electrolyte through at least one duct located in the piercing tool.

The present inventive method makes it possible to eliminate the anode effect or even partly or completely avoid it in a considerably shorter time than by the methods heretofore known and nevertheless obtain the favourable 3 state of the contents of the bath (purification effect) which has heretofore been attributed to an action of the anode effect, so that the disadvantageous consequences of the anode effect do not have to be accepted. This method can be employed both in furnaces with prebaked anodes and in furnaces with self-baking anodes.

By elimination of the anode effect within the definition of the present invention it is understood to mean both the eradication of the completed anode effect, with disappearance of the excess electric voltage, and the prevention and avoidance of the appearance of the completed anode effect, as well as the prevention, or limitation, of the detrimental consequences in its initial period (time of incipient voltage increase).

For example, when the method according to the present invention is employed, the eradication time, that is the duration of the anode effect, is only 20-30 seconds in the case of a complete anode effect of the aluminium electrolysis. The method according to the invention operates in a reliable and quantitatively regular manner. As compared with known proposals with a similar aim, the method has the advantage, inter alia, that it is not tied to specific designs, or divisions of the space, of the electrolytic cell, as is the case with previous proposals for automation and mechanisation, and is capable of general application.

If, in the aluminium electrolysis, we assume a voltage difference of about 26 volts between normal operation (about 4 volts) and anode effect (about 30 volts), a reduction of the anode effect from 5 minutes to 30 seconds, that is by 4 /2 minutes, when the invention is employed, and an average number of 1.5 anode effects per day and furnace, a saving in power consumption of 0.42 kwh./ kg. Al is obtained. The actual reduction of the power consumption is even greater, since the current yield does not remain constant during the anode effect as assumed in the calculation, but is substantially lower than during normal operation. A saving of power of at least 0.5 kwh./kg. Al can therefore be expected.

Various kinds of gases may be employed when the method according to the invention is carried into effect. For example, compressed air gives good results.

The gaseous agent is injected for at least 5 seconds, preferably for more than seconds, at a pressure of more than 2 atmospheres gauge pressure, preferably more than 4 atmospheres gauge pressure.

To what extent the favourable effect of the use of the method according to the invention can be attributed to the intensive swirling action and turbulence of the electrolyte produced by the supply of gas or to those changes which are caused by chemical reactions of the gas introduced or by physical processes, such as removal of the gas film by entrainment (lowering of the partial pressure of the interfering substance causing the voltage increase),

or Whether other processes besides are involved, has not yet become entirely clear, as is also the case with the known methods for eliminating the anode effect. What is a fact is the certainty and the quantitative regularity (voltage drop and shortness of the time taken) of the effect achieved by employing the method according to the invention.

The present invention also includes an apparatus for carrying the method according to the invention into effect, the apparatus comprising at least one mechanically driven tool for piercing the surface crust of the electrolyte, the tool including at least one duct for feeding gas into the electrolyte.

The piercing tool according to the invention, for example, is in the form of a blowing chisel which includes the ducts for the supply of gas in the form of bores or attached tubes. At least that part of the gas ducts which extends in the longitudinal direction of the blowing chisel may also be formed by assembling suitably preformed parts of the blowing chisel. One such chisel or a number of them is or are attached by the upper end to the piston rod of a pressure cylinder or to some other suitable motive means which in turn is mounted, for example, on a crust-breaker truck, or preferably is mounted stationarily on the furnace, for instance on the structure supporting the anode system or else on a support erected on the edge of the furnace tank or beside the latter. The gas feed duet extending in the longitudinal direction of the chisel advantageously has a lateral connection in the upper part of the chisel, below the point where the chisel is attached to the piston rod, and extends into the region of the tip of the chisel, where it may have its outlet at the actual tip, but preferably discharges a little above the tip through laterally directed branch ducts.

The chisel has such a length and such a range of depth that the gas outlet orifices can preferably be immersed to a point below the anodes. Gas can be injected into the electrolyte at such a pressure by means of the apparatus according to the invention than an area of the bath of about 2-4 sq. m., possibly even somewhat larger, is set in motion. For present-day electrolytic furnaces with an anode area of about 10 to sq. m. it is generally sufficient to install 2 to 6 tools per furnace.

The piercing tools may, for example, be arranged at the ends of the furnace, for instance two at each end. This arrangement will be a possibility in particular in furnaces with continuous, self-baking electrodes, or in furnaces with elongated prebaked anodes mounted across the width of the furnace and disposed close to one another. In this case, care should be taken that the gas jets are so arranged in relation to one another that their in dividual effects are additive i.e., as seen from the centre of the furnace, they set all parts of the bath in like rotary motion.

The method and apparatus according to the invention can be employed with advantage in the electrolytic furnaces mentioned in the preamble, the anodes of which are divided into two groups by an interspace provided along the longitudinal axis of the furnace, for example by securing the chisels with gas inlet bores to a beam which extends over the entire length of the anode zone and is adapted to be moved up and down mechanically. In comparison with an arrangement of the tools between the anodes and the edge of the pot, the arrangement of the tools in the longitudinal axis of the furnace has the advantage that gas can be blown out in all directions, which increases the range of action of the individual tools accordingly, since no blowing directions are blocked by walls.

The stationary arrangement of the chisels at the longitudinal sides of the furnace is also possible, but it is frequently desirable that, for maintenance of the furnace and above all for the purpose of exchanging burnedaway anodes, these longitudinal sides of the furnace be as unobstructed as possible.

If, for any reason, it is desirable to stir up the bed of the bath by a whirling action, the blowing orifices of the chisels may be directed downwardly; in this way, the purifying action of an anode effect can be intensified and a purifying action may also be expected when the injection of gas is effected outside the time of an anode effect.

On the other hand, if it is desired that the bed of the bath remain at rest as far as possible, but that the movement of the electrolyte produced by the gas jets extend as far as possible, the blowing orifices are advantageously so arranged that they are directed horizontally or approximately horizontally and blow under the anodes in the lowered position of the chisels.

It has been found, even with blowing chisels arranged in the longitudinal axis of the furnace, that it is more favourable to direct the blowing orifices not parallel to the furnace axes, but obliquely with respect thereto, for example at an angle of -60, preferably 50.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate some examples of apparatus for carrying out the inventive method. In the drawings:

FIG. 1 shows a blowing chisel in longitudinal section;

FIG. 2 is a cross-section through the blowing chisel shown along lines 22 of FIG. 1;

FIG. 3 shows in elevation the lower portion of a modified blowing chisel shown in perspective;

FIG. 4 shows in elevation the lower portion of another blowing chisel also in perspective;

FIG. 5 is cross-section through the blowing chisel shown along the lines 55 of FIG. 4;

FIG. 6a is a longitudinal section through the lefthand half of an electrolytic furnace having an interspace be tween the two longitudinal rows of anodes, the blowing chisels being shown in the raised position; and

FIG. 6b is a similar view of the righthand half showing the blowing chisels in the lowered position.

DETAILED DESCRIPTION The blowing chisel 1 shown in FIGS. 1 and 2 con sists of a solid cylindrical piece, for example of steel, with an axial bore 2 which is worked out from above and is subsequently sealed.

The chisel 1 is secured by means of the overlapping socket 3 to the pistol rod 4 of a pressure cylinder 5. Below the socket 3, the chisel has a lateral bore 6 opening into the longitudinal bore 2 and to which a gas feed pipe 7 is connected. In the vicinity of the tip of the chisel, the longitudinal bore 2 divides into four radial bores 8, which serve as outlet orifices for the gas. In the example shown, these outlet orifices are arranged at right angles to the longitudinal axis of the vertically operating chisel, that is they are arranged horizontally, but, as already mentioned, it is possible to direct these orifices obliquely downwards or slightly upwards. The chisel tip proper, which wears out during operation, may be replaceable.

The blowing chisels are also able to initiate the flow of the alumina lying on the crust of the electrolyte into the electrolyte by means of the holes made by them in the crust. In order to achieve this in an adequate manner, it is advantageous to set a plurality of blowing chisels in action singly or in combination simultaneously or to use multiple blowing chisels. For the purpose of widening the hole which can be made in the crust on the surface of the bath and pushing in a larger amount of alumina, it is also possible to provide the chisel with lateral breaking wings or blades 9 as shown in FIG. 3.

Another example is the winged blowing chisel shown in FIGS. 4 and 5, made from a flat section 10 which is pointed at the bottom and to each of the flat sides of which a longitudinally split tube 11 is Welded, the tube being closed in conical form at the bottom and being provided with blowing orifices 8.

FIG. 6 shows diagrammatically an example of the application of the invention to an aluminium electrolysis furnace the anodes of which are divided centrally into two groups by an interspace extending in the longitudinal axis of the furnace.

The electrolytic furnace consists essentially of a pot 12 with an iron-plate shell 13 in combination with a hollow peripheral reinforcement 14 and iron reinforcing members 15, as well as a brick insulating lining 16 and a cathodic carbon lining 17 in which the cathode bus bars 18 are embedded. In the pot are the layer of molten aluminium 19 and the layer of electrolyte 20, which latter solidifies at its surfaces and at the side walls to form a crust 21 and is covered by the layer of alumina 22. Two rows'of prebaked anodes 2,3 which are disposed close to one another dip into the layer of electrolyte 20. For the sake of clarity, only each two outer anodes 23 of the rear row are shown in the drawing, but not the way in 6 which they are suspended from the anode bus bars, or the bus bars and the further details of the anode system as these are well known.

At the ends of the furnace are the supporting pillars 25 for the anode system, which stand onthe shed floor 24. Mounted on each of these supporting pillars 25 is a bracket 26 directed towards the centre of the furnace, to which brackets there is attached, with the interposition of the two vertically adjustable pressure cylinders 27, a beam 28 extending above the interspace between the rows of anodes. Other thrust-producing means, such as, for example, toggle levers or eccentric drives, may also be used instead of the pressure cylinders 27. In the example illustrated, the beam 28 is equipped with four blowing chisels 1, which are fed through a separate main gas line 29 and the connections 7. This arrangement has the advantage, inter alia, that all the chisels can be actuated together with only two pressure cylinders. The stroke of the pressure cylinders and the length of the individual blowing chisels are so calculated that, in the raised or inoperative position of the beam, the tips of the chisels are located above the crust formed on the electrolyte (left-hand side of FIG. 6) and, in the lowered position of the beam, the outlet orifices 8 of the blowing chisels 1 are located below the anodes 23 (right-hand side of FIG. 6).

Although different ways of accommodating thev apparatus (for example, in mobile fashion on a crust breaker truck or in non-mobile fashion, i.e. mounted on the furnace itself) and different ways in which the apparatus cooperates with the conduct of the electrolysis in other respects may be chosen, a working example will now be described for the case where an anode effect which has become complete is eradicated after an above described cycle of alumina-charging operations.

The electrolytic furnaces in a shed are so operated by feeding them with alumina by means of conventional mechanical crustbreaking devices. that about 1 /2 anode effects are obtained per furnace per day. At intervals of about 4 hours, the surface crust is broken open by portable conventional pneumatic drills, first on one longitudinal side of the furnace tank 12, then on the other longitudinal side and then on the two transverse sides, and in the process alumina is pushed into the bath, after which, after the new crust has solidified, fresh alumina is heaped upon the latter. By suitable metering of this addition of alumina, the anode effect may be provoked when desired. As soon as a furnace indicates the coming anode effect and when the increasing voltage has reached a value of 10-20 v., the blowing chisel apparatus located over the interspace between the two rows of anodes is set in operation, i.e. the beam 28 and its chisels 1 are forced downward under the action of the pressure cylinders 27, the gas feed ducts of the chisels being first fed with compressed air at a small part of the pressure subsequently employed. As the beam descends, the crust is pierced locally by the chisels, the alumina lying thereon is introduced into the melt and the blowing orifices of the chisels are brought below the anodes. The duct system of the blowing chisels is now fed with compressed air at a higher pressure, for example at about 5 atmospheres gauge for 10-20 seconds, which produces a corresponding boiling or bubbling and circulation of the bath. The air consumption is between 2 and 4 cu.m. (measured at N.T.P.)/min. If the time required for the response of the control system and the downward movement of the apparatus (3-5 seconds) is added, the anode effect persists fully for about 10 to 25 seconds. This time has proved in the tests to be fully adequate for purifying the bath. Before the apparatus is raised, the compressedair pressure is again reduced to such an extent that it is suflicient to prevent with certainty any penetration of melt into the blowing orifices. For the purpose of adequate charging of A1 0 care is taken that a sufficiently thick layer of alumina is previously charged on to the crust in the interspace. If necessary, additional pushing may also be carried out two or three times by means of the apparatus to cause still more of the alumina lying on the parts of the crust which have remained solid to flow into the electrolyte at the places which have been broken open and faster flowing in the alumina, it is also possible to employ blowing chisels with a broader piercing surface, for example chisels as shown in FIG. 3 or FIGS. 4 and 5.

During the pushing-in process described and carried out after the anode effect, enough alumina is advantageously introduced into the bath to suffice until the next planned crust-breaking operation, which can be effected by suitable choice of the nature and number of the piercing tools. After a solid crust has again formed on the bath, a Suiciently thick layer of alumina is again placed thereon for heat insulation and to prepare for the next eradication operation.

As can be seen from the example, the method according to the invention solves the problems of effecting the elimination of anode effects and purification of the contents of the bath rapidly, reliably and without manual or mechanical agitation, in trouble-free interplay with the rest of the conventional operation of a furnace, in particular with the supply of the alumina. It is even possible to carry out the supply of the alumina and the eradication of the anode effect in one and the same operation and approximately simultaneously.

In addition, alumina can be introduced into the bath by means of the blowing chisels described during normal operation, i.e. in the periods between the anode effects, so that a separate supplementary crust-breaking arrangement and operation of a crust-breaker or other arrangements known per se for feeding alumina into them are unnecessary.

The operation of the apparatus can be automated with advantage by a programming switching mechanism being switched on, for example by the voltage which is increased on the occurrence of the full anode effect or even earlier by the increase in voltage preceding the anode effects, thus, for instance, at a cell voltage of 20 volts or still earlier, the programming switching mechanism controlling the individual steps of breaking open the crust, immediately following injection of gas and so on.

The method according to the invention also facilitates the full automation of the electrolysis of aluminium which is to be aimed at Gantry-like frames travelling along the series of furnaces are already known which are equipped with crust-breakers and alumina chargers for operation at the sides of the furnaces. Such constructions can be automated, but can only be used for periodic operation, since they are too heavy to be moved rapidly to a furnace showing the anode effect. In these cases, an automatable apparatus according to the invention which is present at the furnace is best suited to intervene without any loss of time, and in fact even when anode effects occur simultaneously in a plurality of furnaces.

As the described automation of the conventional crustbreakers and raw material feeders operating periodically independently of the anode effects means an increased risk of accident for those persons who would moreover have had to carry out besides the manual treatment of the melt which has heretofore generally been customary in the removal of the anode effect, the use of automated apparatus according to the invention also provides a valuable remedy in this respect.

A particularly favourable method of operating an aluminium electrolysis furnace therefore consists in that, in combination, the anode effects are eliminated by applying the method described and the furnace is fed with raw ma terials by known methods between the anode effects.

Thus, it is possible with advantage to feed the furnace periodically, independently of the anode effect, with alumina lying on the crust by breaking the latter, without supplying gas, and produce an anode effect by omitting a periodic charge, whereupon the anode effect is eradicated by breaking the crust in places and injecting gas immediately thereafter at the breaks, using mechanically driven piercing tools having gas adducts. If, at the same time, the periodic charging and the operations for eliminating the anode effect are automated, it is advantageous to control them separately from one another in such manner that the operations take place in response to the occurrence of the anode effect, i.e. in response to an increase in the furnace voltage, whereas the first mentioned process is carried out in a prescribed cycle and at prescribed time intervals. In this way, the essential conditions for full automation of the charging of the furnaces, including the production of the anode effects, are fulfilled.

It is possible to combine the apparatus with conventional crust-breakers, so that both are used to operate a furnace. For example, in the construction according to FIG. 6, solid chisels (without gas ducts) may also be installed between the blowing chisels for the purpose of breaking open a larger area of crust.

The supply of the raw materials, in particular alumina, whether at the time of an anode effect or outside this time, may be carried out in manner known per se, such as by depositing it on the crust of the bath and breaking the crust, by supply and forcing in by means of screws or worms, but it may also be carried out by pnuematic forcing by means of rapidly flowing gases. Thus, it is possible, for example, to inject by means of the described apparatus not only gases, but at the same time also alumina or other substances with which the electrolyte is to be supplied, in that the apparatus is provided with suitable feed ducts or conduits for raw materials or mixtures of raw materials and gas and the entire arrangement is adapted to this purpose from expert points of view. Irrespective of whether the raw materials (alumina and other additions) are introduced by pushing them in or pneumatically, the method according to the invention also has the advantage that, due to the vigorous bubbling and the fine distribution of the raw materials supplied in the melt, which are caused by the gas blown in at the same time, rapid dissolution of these raw materials is obtained and rapid sinking thereof and the undesired formation of undissolved compact crusts of A1 0 on the bottom and on the walls of the bath are counteracted.

We claim:

1. A method of operating a furnace for the recovery of aluminium by fusion electrolysis, wherein the anode effect is eliminated by piercing the crust on the surface of the electrolyte with at least one mechanically driven tool and supplying gas to the electrolyte through at least one duct located in the piercing tool.

2. A method according to claim 1, in which the or each piercing tool is first driven through the crust with a low gas pressure supplied through its duct and alumina is pushed into the electrolyte bath in the process, after which, when the lowered position of the tool has been reached, gas is blown under the anode from the duct for at least 2 seconds under higher pressure and the tool is then lifted out of the bath under low gas pressure.

3. A method according to claim 2, in which, in the lowered position of the tool, the duct is supplied with gas at a pressure of more than 2 atmospheres gauge.

4. A method according to claim 2, in which in the lowered position of the tool, gas is blown under the anode for at least 5 seconds.

5. A method according to claim 1, in which pulverulent raw materials to be used by the electrolysis are mixed with the injected gas.

6. A method according to claim 1, in which, when the furnace voltage has increased to 1020 volts, the surface crust is pierced and the gas is supplied to the electrolyte.

7. A method according to claim 1, in which, between elimination of the anode effects the furnace is fed raw materials.

8. A method according to claim 7, wherein the furnace 9 10 is fed periodically, independently of the anode efiects, References Cited with alumina lying on the crust by piercing the latter, UNITED STATES PATENTS without supplying gas, and an anode effect is produced by 2,432,973 12 7 Smedberg 204 24 omitting or reducing a periodic alumina charge, where- 2,713,024 7/1955 Mantovanello 20467 upon the anode eflect is eradicated by piercing the crust in 5 3,192,140 6/ 1965 Zorzenoni 20467 places and injecting gas through the piercing tool or tools. 3,216,918 11/ 1965 Duclaux 204245 9. A method according to claim 7, in which the opera- HOWARD S WILLIAMS Primary Examiner tions for eliminating the anode efiect and the operations for charging the furnace between the anode eifects are 10 VALENTINE Asslstant Exammer automated, but are controlled separately from one an- US. Cl, X,R other. 204246 

